1. Field of the Invention
The present invention relates to an apparatus for measuring corrosion loss (hereinafter referred to as a corrosion loss measuring apparatus) and to a method for measuring corrosion loss. More particularly, the invention relates to a corrosion loss measuring apparatus for measuring corrosion loss of test pieces formed of a variety of heat-resistant materials such as ceramics and heat-resistant metals under high-temperature gas flow, and to a method for measuring corrosion loss by use of the apparatus.
2. Background Art
In the field of flow of high-temperature combustion gas exceeding 1,000° C. (such situation is found in, for example, a gas turbine), some heat-resistant materials are known to be deteriorated by a combustion gas component. Recent studies have revealed that even a ceramic material (a material having excellent heat resistance) is gradually corroded to lose volume with elapse of time under the flow of combustion gas. One possible reason for the corrosion loss is that steam contained in the combustion gas reacts with the ceramic material to form a hydroxide, and vaporization of the formed hydroxide is promoted by the gas flow, thereby removing a portion of the ceramic material. Therefore, a ceramic material for use in high-temperature (>1,000° C.) combustion gas must be quantitatively assessed in terms of corrosion loss after use for a predetermined period of time, and a simple technique for predicting the corrosion loss amount is demanded.
In order to experimentally induce corrosion loss of a ceramic material, the following three conditions must be satisfied simultaneously. That is, the material is maintained at high temperature higher than about 1,000° C.; the atmosphere gas surrounding the material contains steam; and the material is placed under flow of the atmosphere gas. Notably, the amount of corrosion loss of the material is known to increase with material temperature, partial pressure of steam contained in the atmosphere gas, and gas flow rate.
Conventionally, a combustion-gas-flow-type corrosion loss measuring apparatus is employed for inducing corrosion loss of a ceramic material and quantitatively assessing corrosion loss characteristics of the material. The apparatus is employed in combination with a combustion gas generator such as a burner or a gas turbine combustion apparatus, and a test piece is placed under combustion gas flow. Some types of such combustion-gas-flow-type corrosion loss measuring apparatuses allow independent control of temperature, gas composition, gas flow rate, and gas pressure. However, the above combustion-gas-flow-type corrosion loss measuring apparatuses have drawbacks; e.g., safety is not assured due to combustion of a large amount of fuel; the apparatuses are large-scale and have a complex structure, leading to high cost and mal-operability; and impurities (oxide of a metal member exposed at high temperature, corroded material formed on/in the piping, etc.) contained in combustion gas are deposited on the material test piece. Thus, a limitation is placed on the precision of measurement. In addition, the gas composition can be varied only within a narrow range, and maximum gas temperature is generally limited to about 1,500° C.
Another conventionally employed apparatus is an electric-furnace-type corrosion loss measuring apparatus in which a test piece is placed in a furnace that is electrically heated to high temperature and an atmosphere gas containing steam and simulating combustion gas is introduced into the furnace. As compared with a combustion-gas-flow-type corrosion loss measuring apparatus, the electric-furnace-type corrosion loss measuring apparatus is smaller, is of low cost, is easily operated, and assures high-safety. In addition, the gas composition can be varied over a wide range, and the gas temperature can be readily elevated to about 1,700° C., which are advantageous. However, the composition-regulated atmosphere gas is fed directly into the electric furnace in which the test piece is placed, and a large amount of atmosphere gas must be supplied in order to elevate gas flow rate, resulting in lowering of temperature inside the electric furnace. Therefore, the gas flow rate is required to be limited to some cm/s or less. Under such flow conditions, corrosion loss of the test piece cannot be induced, or even if corrosion occurs, only a small amount of corrosion loss is induced. As a result, it is considerably difficult to quantitatively assess corrosion loss when an electric-furnace-type corrosion loss measuring apparatus is employed.
Meanwhile, in relation to a semiconductor device production apparatus, there have been proposed heating apparatuses for heating, to a predetermined temperature, a gas to be fed to a high-temperature furnace for processing a semiconductor, each heating apparatus being provided so as to heat a conduit for feeding the gas to the furnace. These apparatuses are disclosed in Japanese Application Laid-Open (kokai) Nos. 6-151414, 2001-345314, 2002-277054, etc. Since each of the disclosed heating apparatuses is provided separately from the high-temperature furnace, the heated gas is fed to the high-temperature furnace through a conduit connected to the furnace. Since the gas to be fed to a semiconductor device production device is required to be heated to about 1,000° C., a heating tube or a conduit for feeding the gas supplied from the heating tube may be formed of quartz glass or a similar material.
In the case where a corrosion loss measuring apparatus is employed for inducing corrosion loss of a ceramic material and quantitatively and effectively assessing corrosion loss characteristics of the material, the atmosphere gas is required to be heated to about 1,700° C. Therefore, a heating tube and filler for use in a heating apparatus, as well as a conduit for feeding heated gas, cannot be formed from quartz glass, which is softened at about 1,400° C. A translucent ceramic material such as translucent polycrystalline alumina is an alternative, but the translucent ceramic material is highly expensive. Thus, non-translucent ceramic material such as alumina or zirconia is employed as a material for providing a heating apparatus for attaining a gas temperature of 1,700° C., a heating tube and filler for use in the heating apparatus, and a conduit for feeding heated gas.
When a heating tube and fillers therefore formed of a non-translucent ceramic material are employed, the heat generated from the heating medium is transferred through the inner wall of the heating tube and converted to radiation. The atmosphere gas is effectively heated upon passage through radiated fillers. However, the portion where the gas can be effectively heated is limited to a zone in the vicinity of the inner wall of the heating tube. Thus, when a large amount of gas is fed to the heating tube, a wide temperature distribution profile (i.e., a large difference in temperature) is observed in a cross-sectional area through which the gas flows. When such a temperature distribution profile is observed, reliability of the measured data is considerably affected, and the mean gas temperature at the outlet of the heating tube is lowered. In order to overcome these drawbacks, the heating apparatus must be scaled up, which is problematic. When the measuring furnace in which a test piece is placed is connected with the heating apparatus via a ceramic gas conduit, connection between ceramic parts and sealing the gas in the connection portions become difficult.
Thus, according to conventional techniques, there has not been attained a material corrosion loss measuring apparatus which can induce corrosion loss of a ceramic material through electrical heating and quantitatively and effectively assess corrosion loss characteristics of the material.
As described above, according to the conventional techniques, corrosion loss characteristics of ceramic materials and heat-resistant metallic materials have been evaluated solely by use of a combustion-gas-flow-type corrosion loss measuring apparatus in combination with a combustion gas generator, but unsolved problems in terms of safety, cost, operability, etc. have remained. One candidate apparatus for solving the problems is an electric heating material measuring apparatus. The temperature required for carrying out corrosion loss measurement is about 1,000 to 1,700° C. for ceramic material, and about 800 to 1,200° C. for heat-resistant metallic material. Therefore, the apparatus must employ a ceramic member at high-temperature portions. As a result, in the electric-heating-type measuring apparatus, a large amount of gas cannot be heated effectively and uniformly to about 1,700° C., and sealing of ceramic members for preventing leakage of gas is difficult to attain.